1. Field of the Invention
The invention relates to a laser interferometer used, for example, to measure displacement on a coordinate positioning machine such as a machine tool or coordinate measuring machine, in order to enable calibration of machine parameters such as length, straightness, and squareness.
Laser interferometers include a laser which generates a beam of laser light, a fraction of which is diverted by a beam splitter into a reference arm, typically provided by a retroreflector whose position is fixed relative to the beam splitter. The remaining undiverted part of the beam is incident upon a moving retroreflector, mounted to an object whose displacement is to be measured, and which constitutes the measurement arm of the interferometer. The reflected beams from the measurement and reference arms are recombined at the beam splitter, and the resultant interference beam is directed onto a photodetector. The intensity of the interference beam at the photodetector varies with movement of the movable retroreflector, whose displacement may thus be determined from the cyclically varying output of the photodetector.
2. Description of Related Art
A known problem with such an interferometer, when employed to measure relatively large displacements of the movable retroreflector (e.g. of the order of 100 meters from the laser), is caused by the divergence of the laser beam due to the relatively large distance which the beam has travelled. This beam divergence makes it possible for light reflected from the movable retroreflector to enter the laser cavity from which the outwardly-directed beam was emitted, thus causing unwanted resonances which are deleterious to the measurement accuracy. One known way of avoiding such "back-reflections" is to provide an optical isolator which circularly polarises light passing through the beam splitter (which was, after passage therethrough, linearly polarised) prior to reflection by the moving retroreflector, and, after reflection by the moving retroreflector, to linearly polarise light which is directed toward the laser cavity, thereby enabling the beam splitter cube (which operates upon the basis of a light polarisation) to direct the aforementioned light away from the cavity thus preventing back-reflections. Such a system is known from U.S. Pat. No. 4,844,593.